Reinforced plastic pipe or other tubular articles are normally fabricated by a filament winding operation in which resin coated strands are wound on a mandrel in a series of superimposed layers. After the desired number of layers have been wound to achieve the necessary strength requirements, the resin is cured and the resulting product is stripped from the mandrel.
In the conventional filament winding operation, the speed of winding is determined primarily by the speed at which the strands can be coated or impregnated with the uncured resin. Normally, the strand of reinforcing material is impregnated with resin by passing the strand through a resin bath. If the speed of travel of the strand is too fast through the resin bath, there will not be sufficient impregnation of the fibrous strand. Furthermore, if the resin impregnated strand is wound at too rapid a rate on the mandrel, the liquid resin will be thrown from the strand by centrifugal force and shielding is required to prevent the thrown resin from contacting outside objects.
In the past, attempts have been made to increase the speed of impregnation of the fibrous strands by heating the resin to decrease its viscosity. While the less viscous resin will more readily impregnate the fibrous strands, the heating substantially shortens the pot life of the resin.
A further problem that has arisen in the past with conventional filament winding processes is that the normal operation for impregnating the fibrous strands with resin does not completely eliminate all air or gas from the voids or interstices of the fibrous strands nor does it remove all gas entrained in the resin. If the gas is not completely removed, the mechanical properties of the cured article will be adversely affected.
Conventional filament winding processes are also limited to the use of certain resin systems, such as epoxy and polyester resins, which do not require pressure during the curing stage. Less expensive resins, such as the phenolic resins, cannot normally be utilized because they require pressure during the curing operation.
As a further disadvantage, pipe sections produced by conventional filament winding operations have relatively rough outer surfaces, and the rough surface is a disadvantage when using external coupling members to couple lengths of pipe together. Due to the roughness of the outer surface, it is generally necessary to machine the outer surface of the pipe so that the couplings will seat against a smooth surface.
It is also desirable in a filament wound article to have a resin-rich outer layer which will prevent fiber blooming when the tubular article is subjected to extreme weather conditions. In the normal filament winding operation, the outer surface is not resin-rich, but contains the same proportion of fibers and resin as the remainder of the wall of the pipe. When a pipe of this type is exposed to extreme weather conditions, the fibers at the outer surface tend to "bloom" or break away and become exposed, which is an undesirable characteristic.